The invention relates to a method for the processing of plastic tubes obtained by extrusion, the said method being of the type according to which a plastic tube is brought to a molecular orientation temperature higher than the ambient temperature and is subjected to biaxial stretching by radial expansion and by axial elongation, in order to obtain a bi-orientation tube which has improved mechanical properties.
Such a method is known, particularly from the documents GB-A-1,432,539 or U.S. Pat. No. 4,340,344. The additional processing of the plastic tubes which this method necessitates entails an appreciable increase in the prices of the finished products. However, the great improvement in the mechanical characteristics of the products, in particular increase in mechanical resistance to internal pressure, is conducive to employing this method for products, in particular tubes intended for conveying fluids under pressure, which must have good mechanical resistance.
According to the known method, the tubes manufactured by extrusion are stored at ambient temperature, are subsequently taken from stock and are brought by heating from the ambient temperature to the molecular orientation temperature in order to undergo the processing. The heating of a tube to be processed is relatively lengthy and involves a high energy consumption, since it is necessary to cause the tube to change from the ambient temperature to the molecular orientation temperature. The result of this is that the efficiency of the processing method is not very high and needs to be improved.
The object of the invention is, above all, to provide a method for the processing of tubes made of plastic, especially PVC (polyvinyl chloride), as defined above, which allows higher efficiency, in particular by a reduction in the processing time and by a decrease in the energy consumption necessary for processing.
It is desirable that the method should make it possible to improve the temperature profile in the wall of the tube for the purpose of the biaxial stretching of the latter.
It is desirable, furthermore, that the processing method should remain relatively simple and not require too great an amount of floor space, so that the cost of investments for carrying out the method remains acceptable.
According to the invention, a method for the processing of tubes made of plastic, especially PVC, obtained by extrusion, according to which a plastic tube is brought to a molecular orientation temperature higher than the ambient temperature and is subjected to biaxial stretching by radial expansion and by axial elongation, in order to obtain a bi-orientation tube having improved mechanical properties, is characterized in that:
the tube is cut into elements of specific length when it has just been extruded and when its temperature is higher than the molecular orientation temperature, each element constituting a blank;
each blank is placed in an enclosure in order to be cooled there by a fluid at a temperature near the molecular orientation temperature, the dwell time of the blank in the enclosure being longer than the time for producing the blank by extrusion, in order to allow a correct temperature setting.
Should this dwell time be considered too long, it is possible for the blank to be cooled sharply, but briefly, before it is introduced into the enclosure, by spraying or by means of a bath of cold water. A thin layer of material could then be temporarily at a temperature below the temperature desired for the bi-orientation operation.
The blank is subsequently extracted from the enclosure in order to undergo the processing which ensures bi-orientation.
Preferably, just after being extruded and before being cut, the tube is cooled superficially by passing through a cooling fixture, in order to ensure a clean cut of the still hot tube.
The tube is advantageously cut by means of a shearing operation with a blade device of the guillotine type.
The fluid enclosure is advantageously formed by a tank of hot water which, in the case of PVC processing, is at a temperature near or equal to its boiling temperature at atmospheric pressure.
It is desirable to ensure the best (and uniform) transfer of heat between the fluid and an article of complex shape (the exterior and interior of a tube of great length). For this purpose, preferably, the blanks are agitated in the enclosure. Advantageously, the fluid is circulated in the enclosure, in particular in a direction parallel to the axis of the tube.
The invention also relates to an installation for carrying out the method for the processing of plastic tubes, as defined above.
An installation for the processing of tubes made of plastic, especially PVC, according to the invention comprises an extruder for producing a tube and means for subjecting the tube to biaxial stretching by radial expansion and by axial elongation, in order to obtain a bi-orientation tube having improved mechanical properties, and is characterized in that it comprises:
cutting means at the exit of the extruder, for delivering the tube in elements of specific length when it has just been extruded and when its temperature is higher than the molecular orientation temperature, each cut element constituting a blank;
an enclosure for receiving the blanks and cooling them by means of a fluid at a temperature near the molecular orientation temperature, the enclosure being provided for ensuring a dwell time of the blank longer than the time for producing the blank by extrusion, in order to allow the correct temperature setting,
and means for extracting the blank from the enclosure in order to cause it to pass to the means for carrying out biaxial stretching.
A cooling fixture may be provided at the exit of the extruder, in order to cool the tube superficially and allow a clean cut of the still hot tube.
The cutting means advantageously consist of a cutting-blade device of the guillotine type, in particular fastened to the cooling fixture.
The fluid enclosure may be formed by a tank of hot water which, in the case of PVC processing, is at a temperature near or equal to its boiling temperature at atmospheric pressure.
Means for circulating the fluid in the tank, in particular in the axial direction of the blanks, are provided.
Preferably, the blanks are agitated in the enclosure.
Means for setting in rotation the blank which will be extracted next from the tank may be provided.
Advantageously, a roller conveyor or a belt device is arranged at the exit of the extruder, in alignment with the latter, in order to receive the tube. The speed of the conveyor is substantially equal to the exit speed of the tube from the extruder, in order to reduce the stresses on the emerging tube to a minimum. The speed of the conveyor may be variable around this condition for the purpose of overthickening (or under-thickening) the tube blank locally.
The fluid enclosure for setting the temperature of the blank is preferably placed on one side of the conveyor at a lower level. Pushing means are provided for displacing the blank transversely and causing it to enter the fluid enclosure.